Anhydrous ammonia (NH.sub.3) is commonly used as a soil fertilizer, since it is the most concentrated practical form of nitrogen. In conventional systems, when applying ammonia to soil, ammonia is placed in a tank under pressure. The tank is placed on a trailer and towed by a tractor. Some of the ammonia in the tank vaporizes creating pressure. This pressure is not only used to maintain most of the material in a liquid form, but is also used to force or pump the liquid from the tank.
In particular, liquid ammonia in the tank is pumped through a pressure regulator to a distribution manifold which leads to individual lines. Each individual line is connected to an injection knife which plows through the soil and applies the ammonia. The ammonia, as it travels through the distribution manifold, undergoes a phase change forming a gas which is ultimately applied as a fertilizer.
In order to control application rates, conventional ammonia application systems typically include a controller which adjusts the flow rate of the ammonia in proportion to the ground speed of the tractor or vehicle. The objective of the system is to maintain a desired amount of ammonia applied per unit of land area, usually expressed as pounds per acre or kilograms per hectare. For instance, the vehicle speed can be detected with a radar sensor which sends the speed information to a microprocessor. Based on the ground speed data, the microprocessor calculates the proper flow rate of ammonia necessary to achieve a desired application rate.
The controller can also receive ammonia flow rate information from a flow meter, usually a turbine-type, placed in the supply line between the ammonia tank and the manifold. The flow rate is adjusted by the controller using an electrically-actuated regulating valve also installed between the tank and the manifold. The regulating valve is generally a throttling-type valve that simply closes to restrict flow by increasing the pressure drop across the valve or opens to increase flow by reducing the pressure drop across the valve.
A common problem with the above described system, however, is that the flow rate of the ammonia is difficult to accurately control and measure due to vaporization of the ammonia as it moves through the system. Vaporization of the ammonia can occur as a result of pressure decreases in the line due to emptying of the tank, as a result of warmer ambient conditions, and due to pressure drops that occur in the flow lines. Upon vaporization, the density of ammonia can change over a factor of 200:1. When the ammonia forms a liquid and vapor mixture, it cannot be measured accurately with the flow meter, which is designed only to measure liquid flow rates.
In order to remedy vaporization problems, the ammonia pumped from the pressurized tank can be chilled to a temperature so low that only liquid exists. This has been accomplished in the past by a refrigeration device that vents off a small amount of the ammonia (approximately 1%) flowing through the system. When the liquid ammonia is vented to atmospheric pressure, the heat of vaporization is absorbed from the system, cooling a heat exchanger which contacts the ammonia exiting the tank. The ammonia is then chilled and condensed to a pure liquid and thus can be easily metered. The ammonia vented to atmospheric pressure, on the other hand, can be directed into additional injector knives and applied to the soil.
The above described refrigeration system, however, only insures that the ammonia is in liquid form as it passes through the flow meter. The ammonia still vaporizes as it enters the distribution manifold creating other various problems. Specifically, pressure through the distribution manifold can vary between individual lines due to variable lengths and fittings. Pressure variations in the individual lines cause application rates of the ammonia to vary from knife to knife. This problem is especially magnified at lower pressures and flow rates where greater degrees of vaporization are more likely to occur.
Besides failing to maintain uniformity in the distribution of ammonia between the individual lines, the above described system also only operates within a narrow range of ammonia flow rates. The flow rate of the ammonia is directly tied to the pressure in the system. Specifically, the flow rate of ammonia is proportional to the square root of the pressure. Thus, in order to change flow rates, the pressure must be dramatically increased or decreased. For example, to achieve a 3:1 range of flow control, a 9:1 range of pressure variation is necessary. Pressure variation of the ammonia in the system, however, is limited to the range between the maximum pressure in the tank and the minimum pressure which can maintain the ammonia in a liquid state. Consequently, wide ranges of application rates are simply not practically possible.
Conventional systems as described above also have very slow response times due primarily to the slow actuation of the throttling valve used to control pressures and flow rates. Typical valves used in these systems can take between 1.5 to 10 seconds to change position.
Prior art systems have also failed to provide individual control of the flow of ammonia to each knife. As described above, the flow of ammonia is only controlled as it enters the distribution manifold, prior to the ammonia branching off into each individual line.
Thus, a need exists for an improved system and process for injecting volatile liquids, such as ammonia, into soil. In particular, a need exists for an ammonia application system that uniformly applies ammonia to soil, operates at a wide range of flow rates, offers fast response times when changing flow rates, and allows the ammonia application rates to be varied between the individual distribution lines.